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Becht Engineering Blog

In this section of the site contributing authors submit interesting articles relating to the various services, industries and research & development efforts of Becht Engineering.

Resolution of Chronic Cooling Tower Fan Vibrations

Resolution of Chronic Cooling Tower Fan Vibrations
Traditional rotating equipment mounted at grade use the mass of a foundation and grout to reduce vibration and provide support and stiffness. However, cooling tower fans must be elevated many feet above a basin of water which sometimes puts them “out of sight and out of mind”. Cost and practicality prohibits concrete and grout, thus most cooling towers are constructed by bolted wood and / or fiberglass elements with a fabricated steel sub base supporting motor, gearbox, and blades. As a result, cooling tower foundations are much less stiff than traditional rotating equipment. This is usually not an issue because the low speed of the fan (usually 60 rpm to 140 rpm) does not usually produce large forces. Motors and gearboxes used are generally made to the same precision levels as other general purpose machinery regarding balance, runout and dimensional tolerances, so the imbalance and forcing functions are usually low.  As...
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“Speed Kills” – An Empirical Association of Antifriction Bearing Behavior When Things Go From Bad to Worse

“Speed Kills” – An Empirical Association of  Antifriction Bearing Behavior When Things Go From Bad to Worse
Machinery and reliability people have for a long time observed that when it comes to long life and high reliability, operating at higher speeds compares poorly to operation at lower speeds. One of the primary reasons for having high-speed devices is economics. With high speed you can have smaller equipment sizes with fewer stages. While capacity and pressures developed do improve, this often occurs at the cost of increased wear, vibration, noise and maintenance. There are some limits as to how fast and large we can practically go. At some point even the required lubrication systems become very complex. While smaller equipment can use antifriction/rolling element bearings, their application is typically limited by manufactures to a certain speed. When combined with the bearing size, this limit becomes known as a DN number limit, a product of the mean bearing diameter (ID+OD in mm divided by 2) and the RPM. This DN...
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No Detail or Procedure is Trivial, or, Sophisticated Vibration Analysis is a VERY Cost Effective Diagnostic Tool

No Detail or Procedure is Trivial, or, Sophisticated Vibration Analysis is a VERY Cost Effective Diagnostic Tool
Today it is common practice in overhauls of large rotating equipment to spend many man hours in detailed planning, coordination, writing procedures, reviewing documentation, reviewing past photos, procuring and inspecting spare parts etc. We also review our past experience and put steps in place to mitigate past problems or errors.   Now that we have a detailed plan, have purchased and inspected all necessary spare parts and have mitigated all potential known problems we simply give the plan to the machinery contractor to execute, right? If you have been responsible for large machinery repairs long enough, you have definitely learned to say one thing: “I have not seen everything,” and never to say another “Oh that could never happen here.” No matter how thorough we think our plan is, going in, missing even the slightest detail in the field can result in a failed start up or worse. Experienced oversight is essential...
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When Should the Rules for Severe Cyclic Conditions (Service) in ASME B31.3 Be Used?

There has been a fair amount of confusion as to when the rules for severe cyclic conditions in ASME B31.3 should be used, and the rules themselves can be somewhat confusing to apply.  The definition as to when the rules of severe cyclic apply is in the 300.2, Definitions.  It states that it is severe cyclic conditions are: • Conditions applying to specific piping components or joints in which SE computed in accordance with para. 319.4.4 exceeds 0.8SA (as defined in para. 302.3.5), and • the equivalent number of cycles (N in para. 302.3.5) exceeds 7000; • or other conditions that the designer determines will produce an equivalent effect. So, severe cyclic conditions applies to piping systems with a lot of displacement cycles, which are rare in most process plants, and the calculated displacement stress is close to the allowable displacement stress.  For these piping systems, fatigue is a greater concern. Following the rules...
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Recent Comments
Chuck Becht

answer to question

There is an new Appendix being voted on for high cycle fatigue, which is suitable for use with FPSO's. However, I believe it's fo... Read More
Wednesday, 01 October 2014 16:25
Chuck Becht

severe cyclic

Severe cyclic applies when the number of cycles exceeds 7000 so the f factor will be 1 or lower, not 1.2. The 0.8 factor is a c... Read More
Thursday, 04 February 2016 12:01
Chuck Becht

Response to recip piping comme...

The B31.3 code has changed the criteria to a more subjective one, and I will address that in a future blog. But it does not speci... Read More
Monday, 09 October 2017 09:34
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